FIG. 1 illustrates a three phase auto transformer 10 having first, second and third primary windings 12, 14 and 16 which are respectively magnetically coupled to secondary windings 18, 20 and 22. The secondary windings 18, 20 and 22 are in a delta connected tertiary which is idle under normal balanced operation but contains current flow when a fault or an unbalanced load situation exists in electrical loads (not illustrated) connected to the phase outputs 24, 26 and 28. Phase inputs are applied to terminals 30, 32, and 34.
FIGS. 2A and B illustrate respectively a three phase auto transformer an a zig-zag transformer with windings 13, 15 and 17 connected to phase inputs and outputs 30, 32 and 34 and ground which is known as an interconnected star grounding transformer. In FIG. 2A each of the windings 13, 15 and 17 has a first part 19 wound in a first direction and a second part 21 wound in the opposite direction. In FIG. 2B, each of the windings 13, 15 and 17 has a first part 19 and a second part 21 wound in the same direction. In FIGS. 2A and B, the windings 13, 15 and 17 function to divide the neutral current into three equal parts when the transformer is connected to an unbalanced load (not illustrated).
U.S. Pat. No. 4,617,622 discloses a three phase inverter which has three pairs of switches connected between reference points of a DC potential. The switches are switched to produce a three phase AC output. The three phase output is filtered. The filtered output is connected to a neutral forming transformer.
A power supply has been developed by Westinghouse Electric Corporation having a switching matrix which switches a DC potential coupled to the switching matrix to create a staircase waveform at a desired fundamental frequency to produce three phase AC. With reference to FIG. 3 of the present invention, the prior art Westinghouse topology has a switching matrix containing switches Q.sub.1, Q.sub.1, Q.sub.2, Q.sub.2, Q.sub.3 and Q.sub.3 which are switched to provide a three-phase output which is connected to a filter and a neutral forming transformer. For an unbalanced load, current flows from one of the switches Q.sub.1, Q.sub.2 and Q.sub.3 which is conductive through the load, the neutral, and back to the negative terminal of the DC source through two of the switches Q.sub.1 Q.sub.2 and Q.sub.3 which are not connected to a conductive switch. This power supply does not utilize a fourth wire to return current flowing through the neutral to the DC source.
U.S. Pat. No. 3,775,663 discloses that a problem has existed regarding inverters with a neutral terminal in maintaining the neutral terminal at correct potential when loads are unbalanced. The '663 patent further discloses that inverters with three output terminals have used a polyphase output transformer connected, for example, delta on the inverter side and wye on the load size. This produces a neutral transformer terminal which is undisturbed by unbalanced loads. The '663 patent further discloses that the pulse width modulation of each phase of an inverter to produce three phase output current. Each inverter phase consists of a pair of switches connected between two reference potentials which are alternatively pulse width modulated such that each switch is closed to conduct current from one of the reference potentials through a phase output which is filtered to eliminate higher frequency components than the fundamental desired phase output. Furthermore, the configuration of the inverter requires that the potential which is to be applied to the load must be switched completely by the switches of the inverter. If a high potential is required for the load, it is necessary for the switches to switch this high potential which can cause damage or failure.
U.S. Pat. No. 4,564,895 which is assigned to the assignee of the present invention discloses a bidirectional switch which connects an inverter to the neutral.
U.S. patent application Ser. No. 128,444, filed on Dec. 3, 1987, which is assigned to the assignee of the present invention, discloses a DC to AC inverter with neutral in which the neutral is formed by a filter which functions to shunt high frequency harmonics to the neutral of the inverter and an envelope defined by the fundamental frequency being applied to a three phase load with neutral. The inverter has first, second and third switching circuits each having a series circuit having first and second switches with at least one of the first switches and at least one of the second switches being in the on state during operation of the inverter in driving an unbalanced load to permit current to flow from the LC circuit to the unbalanced load and from the unbalanced load back to the LC circuit without a fourth wire.
In airframes it has been the practice to run three phases and a fourth wire which is a neutral from the generator to the load. The fourth wire adds considerable weight as a consequence of a large diameter necessary to permit the flow of substantial neutral current when the three phase load is substantially unbalanced. This fourth wire represents a weight penalty.